1. Field of the Invention
The present invention relates to a substrate for a magnetic recording medium, a magnetic recording medium, a method of manufacturing a substrate for a magnetic recording medium, and a method of inspecting a surface of a substrate for a magnetic recording medium.
2. Description of Related Art
With the recent increase in demand for hard disk drives (HDD), a magnetic recording medium built in the HDD has been manufactured more and more actively. An aluminum substrate and a glass substrate are widely used as a substrate for a magnetic recording medium. The aluminum substrate has a feature that it is high in workability and is low in cost. On the other hand, the glass substrate has a feature that it is superior in strength. Particularly, requirements for a decrease in size and an increase in density of a magnetic recording medium have recently increased and there is a need for reducing the surface roughness or undulation of the surface when manufacturing the substrate for a magnetic recording medium.
When polishing the surface of such a substrate for a magnetic recording medium, for example, a sun-and-planet gear type polishing machine is used which includes an upper platen and a lower platen pinching a substrate (a polishing object), a lower platen support supporting the lower platen, and a polishing solution supply unit supplying a polishing solution between the upper platen and the lower platen and in which a sun gear protrudes from a hole formed at the center of the lower platen (see Japanese Unexamined Patent Application, First Publication No. 2009-6423). Japanese Unexamined Patent Application, First Publication No. 2009-6423 discloses a configuration in which discharge means for discharging the polishing solution remaining in at least one of the center side of the lower platen and the top surface of the sun gear to the outside is provided to reduce the surface roughness of the polishing object.
Regarding the surface roughness of a magnetic recording medium, it has been proposed that the roughness intensity PSD (Power Spectral Density) of a wavelength of 1 to 5 μm in a spectrum of the surface roughness measured with an atom force microscope (AFM) is set to 0.5 nm2 or less and the PSD of a wavelength of 0.5 μm or more and less than 1 μm is set to a range of 0.02 to 0.5 nm2 (see Japanese Unexamined Patent Application, First Publication No. 2001-67650). Japanese Unexamined Patent Application, First Publication No. 2001-67650 discloses that the running durability of the magnetic recording medium can be improved by satisfying the above-mentioned numerical range.
Moreover, the power spectrum density (PSD) indicates the surface structure of a substrate which is decomposed into components for each spatial frequency f (a reciprocal of a space period L of undulation) and which is expressed as the density of each component (see Japanese Unexamined Patent Application, First Publication Nos. H09-152324 and 2006-194764).
The relationship between the PSD as the surface structure of a substrate and the root mean square roughness (Rq: old RMS) can be expressed by Expression 1.Expression 1Rq=√{square root over (∫PSD(f)df)}  (1)
A method of detecting a signal generated through a contact of a detection element disposed in a protrusion inspection head with protrusions by changing a degree of fly of a protrusion inspection head depending on a variation in temperature in a state where the protrusion inspection head is flied from a rotating magnetic recording medium has been proposed as a protrusion inspection method of a magnetic recording medium (see Japanese Unexamined Patent Application, First Publication No. 2004-185783).
In such a magnetic recording medium used in an HDD or the like, there is a need for reducing a distance between the surface of the magnetic recording medium and a magnetic head more than before in response to the requirements of the market for improvements in the recording density.
However, in order to reduce the distance between the surface of the magnetic recording medium and the magnetic head, it is necessary to enhance the surface smoothness of the substrate used for the magnetic recording medium. However, when the surface accuracy of the polishing platen is enhanced, the grain size distribution of polishing grains is sharpened, the number of polishing steps in a polishing process is raised, and the polishing time is increased, the manufacturing cost of the substrate markedly increases.